1. Field of the Invention
The present invention relates to a gas sensor element and a gas sensor for detecting the concentration of a specific gas component in a gas to be measured.
2. Description of the Related Art
Known gas sensors include those for detecting the concentration of a specific gas component in a measured gas such as exhaust gas discharged from an automobile. This type of gas sensor employs a gas sensor element, the electrical characteristics of which change with the concentration of the specific gas component in the measured gas. This gas sensor element is provided with a solid electrolyte member, which is composed mainly of zirconia, for example. Such known gas sensor elements have an overall plate-like outer shape made by laminating one or more solid electrolyte members, electrodes, an insulating layer, a heater and the like. The heater is constituted by laminating an insulating layer, e.g., a ceramic layer composed mainly of alumina, a heating resistor and the like.
In the gas sensor element described above, one end portion (or the leading end portion) in the longitudinal direction of the plate shape is provided as a detecting portion for exposure to the gas to be measured, and the other end portion (or the rear end portion) is fixed on the main fitting. As such, known gas sensor elements (as described in JP-A-2003-294687, for example) have a reference electrode and a detecting electrode disposed on the two faces of the leading end side of the solid electrolyte member formed in a plate shape, and lead portions connected to those electrode portions which are individually formed along the longitudinal direction of the solid electrolyte member.
It is also known (as described in JP-A-2002-202280, for example) that the activation time of the gas sensor element (i.e., the time period required for the sensor to become active after starting the supply of electric power) can be shortened by setting the area of the detecting electrode 1.25 times or more than that of the reference electrode.
3. Problems to be Solved by the Invention
In the gas sensor element described above, however, due to tolerances or the like in the manufacturing process, a pair of electrode portions (e.g., the reference electrode and the detecting electrode), as formed on the surface and back of the solid electrolyte member, may be formed with a slight deviation in position from one another. If the reference electrode and the detecting electrode are formed such that their relative positions deviate, the effective area of the electrode portions which contribute to the detection of the specific gas (that is, the area of the respective electrode portions which directly oppose one another) differs for each gas sensor thus manufactured. Thus, a problem arises in that the gas sensors vary in performance from one another.